Immersed membranes are a class of filtering membranes. The membranes may have pores in the microfiltration or ultrafiltration range. Individual membranes may be assembled into modules that provide a port or fitting for making connections, for example to withdraw permeate, to one or more membranes. The modules may be assembled into cassettes. The cassette may provide a frame to hold several modules together at a desired spacing or orientation between them. The frame may also support one or more headers or manifolds, for example a permeate manifold for connecting a permeate port for each module to a common permeate port for the cassette. The cassettes may be immersed in tanks filled with water to be treated at ambient pressure and connected to a source of suction for withdrawing permeate through the membrane walls. In large plants, one or more cassettes may be grouped together into larger production units that may be called trains. Multiple trains of cassettes may be located in a set of distinct tanks. The arrangement of the cassettes, trains and other components of the plant, the configuration of pipe, valve and pump networks for permeation and other functions, and the process steps for operating the plant can each have significant effects on the overall efficiency of the plant, its tolerance of equipment failures or its sense of operation.
U.S. Pat. No. 6,214,231, issued Apr. 10, 2001 to Cote et al., described a plant in which plural cassettes are grouped together for operation in parallel. One or more of the cassettes may be isolated while permeate production from the remaining cassettes is uninterrupted. Ancillary functions such as backpulsing, chemical cleaning and integrity testing may proceed in the isolated cassette or cassettes while permeation continues with the other cassettes. U.S. Pat. No. 6,214,231 is incorporated herein in its entirety by this reference to it.